allow explicit GITHUB_USER and GITHUB_PASS

when migrating a private repo from an organization, the following error occurs:

    gitea-github-migrator_linux_amd64 migrate --gh-token $GITHUB_TOKEN --token $GITEA_TOKEN --owner 1 --gh-repo $ORGA/$REPO --url $GITEA_SERVER

    panic: error while migrating repo[ORGA/REPO]: Clone: exit status 128 - fatal: could not read Username for 'https://github.com': No such device or address

This patch fices htat by allowing to infuse the correct credentials like this:

    env GITHUB_PASS=$GITHUB_TOKEN GITHUB_USER=jnweiger ~/go/bin/gitea-github-migrator migrate --gh-token $GITHUB_TOKEN --token $GITEA_TOKEN --owner 1 --gh-repo $ORGA/$REPO --url $GITEA_SERVER

Not the nicest implementation, but this gets me going. Thank you for a great tool!

cmd/migrate.go

@@ -1,6 +1,7 @@
 package cmd
 
 import (
+  	"os"
 	"context"
 	"fmt"
 	"strings"
@@ -54,6 +55,12 @@ func runMigrate(ctx *cli.Context) error {
 
 	username := strings.Split(ctx.String("gh-repo"), "/")[0]
 	repo := strings.Split(ctx.String("gh-repo"), "/")[1]
+  	if u := os.Getenv("GITHUB_USER"); u != "" {
+		m.AuthUsername = u
+    }
+    if p := os.Getenv("GITHUB_PASS"); p != "" {
+    	m.AuthPassword = p
+    }
 
 	return migrate(c, gc, m, username, repo, ctx.Bool("only-repo"))
 }